Methods for manufacturing structured layers based on heat-stable polymers are known, for example from the German Patent document 2 308 830 as well as the European Patent documents 0 019 123 and 0 026 820. In these methods, soluble photoreactive intermediary stages of highly heat-stable polymers are used for photolithographic structuring, and the structures produced from them cyclized in a subsequent tempering step. For the complete cyclization and removal of the cleavage products temperatures of up to 400.degree. C. are required. This requires substrates capable of withstanding high thermal loads.
In the fields of circuits and conductors, e.g., in circuitboard technology, substrates based on epoxides are used, which can be thermally stressed up to maximally approximately 150.degree. C. per hour, and which need to be able to tolerate temperatures of approximately 280.degree. C. only for seconds, for instance during soldering processes. The solder resist lacquers used here to partially cover conducting paths must meet similar thermal demands; in order to cover the sites of the circuit surface which are not in contact with the soldering metal, polymers of medium thermal stability are required. The dry resists used until now for this purpose, e.g., silk screening lacquers, based on epoxides and acrylates, meet the requirements of the function of a solder resist mask. However, these resists only partially fulfill the increased demands relating to measuring accuracy (pattern outline accuracy) in microelectronics technology with structures of less than 100 microns, as well as the requisite cycling strength. To this end, photolithographic lacquer systems are required.
A photostructurable lacquer system based on epoxides with chalcon groups built into the polymer chains, i.e. groups having the structure --C.sub.6 H.sub.4 --CH.dbd.CH--CO--C.sub.6 H.sub.4 --, is already available, which would ensure sufficient measuring accuracy (see "Chimia", Vol. 38 (1984), pages 13 to 20). With respect to the photostructurability, however, relatively long exposure times, and particularly long developing times are involved. Beyond that, with the known lacquer systems, additional permanent protection of the circuit surface (which is often sensitive to damaging gases) can only be achieved through elaborate multiple coating processes. Furthermore, the overall process, is long and cost-intensive, due to the lengthy after-curing times, which sometimes last for several hours.
Accordingly, an object of the invention is to develop a method for manufacturing precisely dimensioned, high-quality, heat-stable structured layers, in particular on circuit surfaces, in a single coating process, which also resist the extensive thermal and mechanical stresses, for example, during immersion soldering processes, and, which moreover, protect the circuit surfaces effectively and permanently against moisture and corrosion. In particular, the overall process is intended to be shortened by allowing shorter exposure, developing and tempering times and thus become more cost-effective.